Background High‐quality data on bone marrow involvement (BMI) assessed by flow cytometry (FC) in follicular lymphoma (FL) is lacking. Aims We set up a prospective protocol with a 10‐color tube and acquisition of 500.000 leukocytes on a Nav flow cytometer for evaluation of BMI in FL by FC. Materials and methods FC was compared with a combination of histopathology and IGH gene rearrangement, which were considered the gold standard. We also compared BMI by FC with PET. Results Fifty‐two patients were included (median 67 years, 54% female). BMI by FC was seen in 35 (67%), with a median involvement of 1.2% (interquartile range: 0.3%–7%) of leukocytes. Comparison with the gold standard revealed two false negatives and two false positives (potentially true involvement undetected by the gold standard). BMI by PET was seen in 14/46 (30%). Immunophenotype of FL in the bone marrow was highly heterogeneous. The most common phenotypic abnormality was dim expression of CD19 (>0.5 log loss in 30% of patients). CD10 was negative in 13 (37%) and incompletely positive (overlap with the negative population) in a further 8 (28%) while entirely positive only in 14 (48%). Other abnormalities (loss of CD20, gain or loss of CD79b, expression of CD43, and substantial loss of CD45) were rare. Computational analysis by means of FlowSOM confirmed the heterogeneous phenotype, with FL from different patients clustering in unrelated metaclusters. Conclusion BMI by FL was frequent and immunophenotype was heterogeneous. However, this protocol enabled detection of FL in bone marrow in the vast majority of patients with bone marrow involvement by the gold standard.

A 64-year-old man was sent to the haematology outpatient clinic for macrocytic anaemia (haemoglobin 125 g/L [normal 135-175 g/L], mean corpuscular volume 104 fL [normal 80-95 fL]), neutropenia (0.9 Â 10 9 /L [abnormal<2 Â 10 9 /L]), persistent monocytosis(1.9 Â 10 9 /L [abnormal>0.9 Â 10 9 /L], 30% of leukocytes) with normal platelet count (245 Â 10 6 /μL). Granular and platelet dysmorphia were seen in the peripheral blood (PB) smear. Serum lactate dehydrogenase was elevated (476 U/L [normal 105-333 U/L]). A PB sample was sent for phenotypic characterization of the monocytes (Figure 1). A major discordance between CD33 bright and CD14-positive events was seen, which, after excluding major expansions of other CD33 bright cells, raised the suspicion of paroxysmal nocturnal hemoglobinuria (PNH).This was confirmed (Figures 1 and 2). CD56 was expressed in 40% of monocytes. The patient was asymptomatic. The bone marrow smear revealed increased cellularity with 91% dysgranulopoiesis, 14% dysmegakaryopoiesis, 1% dyserythropoiesis, 8% monocytes, 2%promonocytes and 1% blasts. G-banding karyotype was 46,XY [20].

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